Characterization of Motility and Erythrocyte Adherence as virulence factors in African trypanosomes

Abstract

Pathogens causing African animal trypanosomiasis (AAT), the major livestock disease in subSaharan Africa, belong to the salivarian group of the African trypanosomes, which are transmitted by the bite of the tsetse fly (Glossina spec.). T. vivax, T. congolense and T. brucei brucei are major pathogens of cattle in particular, causing nagana, with dramatic socio-economic consequences for the affected regions. The parasites additionally have a huge reservoir of other livestock and wild animal hosts. T. brucei, the species which also includes the subspecies pathogenic to humans causing sleeping sickness, has been extensively studied as the ultivatable model trypanosome. But less is known about the other salivarian species, which are not routinelyheld in culture, if at all possible. A hallmark of trypanosomal lifestyle is the protozoan flagellates incessant motility, which enables them to populate an enormous range of habitats in very diverse hosts. We were now able to characterize, for the first time with high spatiotemporal resolution microscopy, the swimming behaviour and mechanism of the most relevant salivarian species isolated directly from blood. We show the influence of viscosity on the motility of bloodstream form (BSF) cells and simulate their movement between erythrocytes, giving a clear picture of how all analyzed species move under varying environmental conditions. We show that although the basic mechanism of flagellar motility applies to all analyzed species, there are clear morphological differences that produce different reactions to the physical environment. We could define specific conditions for highly increased swimming persistence and speed for compared to the behaviour in standard culture. These results have important implications for the parasites survival strategies in the host, e.g. regarding the capacity for antibody clearance. Although we show all species to effectively remove antibodies from the cell surface, T. congolense differed markedly in its motility behaviour, which gives rise to interesting questions about this species behaviour in the bloodstream. Most of the T. congolense parasites (and to a lesser extent T. vivax)adhere to sheep erythrocytes. Further in vitro studies showed that T. congolense and T. vivax adhered to rabbit, goat, pig and cattle erythrocytes- but binding behaviour was absent in murine blood. Notably, both T. brucei and T. evansi lacked adherence to all studied host erythrocytes. Generally, attachment to blood cells caused reduction of swimming velocities. Judging from its cell architecture, as well as the motility studies in higher media viscosity and in micropillar arrays, T. congolense is not adapted to swim at high speeds in the mammalian bloodstream. Low swimming speeds could allow these purely intravascular parasites to remain bound to the host erythrocytes.